The curious robotic creation pictured below is called the Roomba QuadDrive, and hails from the University of Bonn's Autonomous Intelligent Systems Lab (NimbRo@Home). The Roomba QuadDrive is an omnidirectional mobile robot base of the powered caster variety with one obvious (whimsical) characteristic: it uses four IRobot Roomba 530s as the drive motors, each paired with a Robotis Dynamixel RX-64 servo for steering! Another of the lab's robots, named Robotinho, surfed atop the QuadDrive base when competing in the 2009 RoboCup@Home competition and when it gave tours of the Deutsches Museum in Bonn -- a short clip of which is embedded below.
According to the NimbRo@Home webiste, the QuadDrive can support a 20kg payload and achieve a maximum speed of 0.5 m/s. To me, this sounds like a perfectly reasonable use for all those unused Roombas lying around -- afterall, any robot built upon a QuadDrive will necessarily help the home clean. Anyway, here are some higher-resolution pictures of the Roomba QuadDrive, including one of Robotinho "surfing" (from RO-MAN 2009):
A video of the surfing action can be seen in the video below (around time 2:00).